CTLA-4 was first described as a negative regulatory molecule by our group in 1994. During the past 11 years the molecule has been extensively studied. The molecule has been shown to have an intrinsic effect on T cell activation by directly delivering negative signals to T cells to shut down activation and differentiation. In addition, there have been a number of studies suggesting an extrinsic role for CTLA-4 as the molecule has been proposed to be the effector molecule by which regulatory T cells suppress immunity. All of these "functions" have been complicated by the recent discovery that CTLA-4 can be expressed as a B7 ligand non-binding variant that controls T cell activation. In addition, polymorphisms in the CTLA-4 gene has been linked to Graves Disease and Type 1 Diabetes. Thus, in this renewal, we will continue to addreess the fundamental biology of CTLA-4. We propose to directly address both the intrinsic and extrinsic role of CTLA-4 in the regulation of initial T cell activation, ongoing autoimmune responses and maintenance of tolerance. The following specific aims are proposed: 1. To study the biochemical basis of B7 ligand-dependent and ligand-independent CTLA-4-mediated inhibition of T cell function;2. To study the intrinsic role of CTLA-4 in T cell development, regulation of tolerance induction and development of diabetes in NOD mice;and 3. To study the intrinsic role of CTLA-4 in T cell development, regulation of tolerance induction and development of diabetes in NOD mice. We will use a combination of novel mice and reagents, combined with TCR transgenic and bone marrow chimera models to identify the cellular and mechanistic basis for CTLA-4 regulation. The results of these studies will provide insights into the mechanisms of CTLA-4 regulation of immunity, and test the the role of CTLA-4 in lymphoproliferative/homeostasis versus immune activation and tolerance are mediated by distinct extrinsic versus intrinsic pathways. Importantly, the information learned from this study may have important implications for public health for several reasons. T cells play a central role in immunity and autoimmunity. As such, any means to modulate T cell activity may lead to novel therapeutic approaches to treat these disease. Moreover, the fundamental importance of CTLA-4 in the induction and maintenance of peripheral tolerance is central to our understanding of disease etiology and current therapeutic approaches.